Parts for your 2018 Mazda 3-Temperature sensors

2018 Mazda 3 temperature-sensors: what they do and how to look after them

Temperature-sensors absolutely are used on the 2018 Mazda 3 (BN series, Skyactiv-G). Mazda’s BM/BN Workshop Manual and wiring diagrams list several, including the Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor (integrated in the MAF), ambient/outside air temperature sensor for the HVAC and display, and a transmission fluid temperature sensor on automatic models. These sensors feed the PCM/ECM and HVAC modules to manage fuelling, ignition timing, cooling fans, AC performance and the dash gauge.

On this Mazda 3, the ECT sensor is the star player for engine protection. It tells the ECU how hot the coolant is so the car can adjust fuel on cold starts, switch cooling fans on, and pull timing if temps climb. The IAT sensor keeps tabs on the air going into the engine—vital for accurate fuel trims and knock control. Up front near the grille, the ambient sensor helps the AC behave properly and gives a fair outside temperature reading. Auto models also monitor transmission fluid temperature to look after shift quality and gearbox longevity.

There’s no routine replacement interval for these sensors, they’re generally “fit and forget”. Good servicing is mostly about prevention:

• Keep coolant fresh and correct (Mazda FL22 or as specified). Old coolant can contaminate the ECT sensor and skew readings.
• Inspect connectors and wiring during services—look for corrosion, broken tabs or rubbed-through looms, especially at the front bumper (ambient sensor) and around the intake (MAF/IAT).
• Make sure the radiator fans cycle normally and the temperature gauge behaves as expected.

Signs a sensor’s gone a bit crook include hard cold starts, rough idle, poor fuel economy, radiator fans running constantly, a dead AC on hot days, or a temp gauge that doesn’t make sense. A quick scan for fault codes (P0116–P0119 for ECT, P0111–P0113 for IAT, for example) and live-data checks will usually pinpoint the culprit.

When replacing, work on a cold engine. For the ECT sensor, safely relieve pressure, catch and reuse/replace coolant as required, and fit the new sensor with the correct seal. Don’t use thread tape unless the manual specifies it. Torque to Mazda spec, refill and bleed the cooling system, clear DTCs, and recheck for leaks. For the IAT on this model, it’s part of the MAF—so replacement is of the full MAF assembly. Ambient sensors are simple plug-in units behind the bumper, handle gently and clip the harness back exactly as routed. If you’re unsure, follow the Mazda 3 BM/BN Workshop Manual procedures or use a qualified tech.

FAQs: 2018 Mazda 3 temperature-sensors

Where are the temperature-sensors located on a 2018 Mazda 3?

The ECT sensor sits on the engine’s coolant outlet/thermostat housing area. The IAT sensor is built into the MAF on the air intake near the airbox. The ambient/outside air temp sensor mounts behind the front bumper near the grille. Automatic models also read transmission fluid temperature from an internal sensor within the transmission assembly. HVAC systems have additional sensors, like the evaporator temperature sensor inside the cabin unit.

What are common symptoms of a failing coolant temperature sensor?

Look for hard cold starts, high fuel use, rough idle, black smoke or fuel smell, radiator fans that run constantly, erratic temp gauge behaviour, or AC cutting out on hot days. A scan tool often shows codes such as P0116–P0119, and live data may be implausible (stone-cold engine reading very hot, or never warming up).

Do temperature-sensors need routine replacement, and what might it cost?

They’re not a scheduled item—replace only if faulty. In Australia or New Zealand, an ECT sensor typically costs roughly $40–$150 plus 0.5–1.0 hours labour, depending on access and coolant handling. The IAT is integrated in the MAF, so that part is pricier. Ambient sensors are usually inexpensive and quick to fit. Always confirm with a proper diagnosis before replacing parts.